The present invention relates to the field of signal processing, and in particular to apparatus and method for determining the energy of a signal.
An ideal exponentially weighted root mean square (ERNS) detector for determining the energy of an analog signal s(u) is well known from the theory of electrical signal processing. The energy may be determined sequentially by executing three method steps: squaring the signal, integrating the squared signal, and extracting the root of the integrated signal. These method steps are also reflected in the following equation: ##EQU3##
which describes the functional principle of the ideal analog detector. The detector determines the energy y of the signal s(u) as its RMS value, weighted exponentially with a time constant T, as a function of time t.
The conventional digital ERMS detectors receive a digital signal sn at their input in order to deliver a digital energy signal yn from the output, with the amplitude values of the energy signal representing the energy of the signal sn. They are based on method steps known from theory. To convert these method steps, as a rule as shown in FIG. 4a, they are formed as a series circuit consisting of a squaring element 1, a low-pass filter 2, and a root extractor 3.
FIG. 4b shows one possible digital implementation for such a series circuit. Accordingly, squaring element 1 is formed from a first multiplying element 410 that multiplies the digital signal sn by itself in order to provide the squared signal s.sup.2 n and its output. The squared signal is then supplied as the input signal to the digital low-pass 2 weighted with a factor tau.
Within the low-pass 2, the input signal is fed as a first summand to an adding element 420 which delivers at its output the desired energy signal but squared as y.sup.2 n. As the second summand, the output signal y.sup.2 n fed back through a state memory 430 weighted with the factor (1-tau) is supplied to adding element 420.
Then the squared energy signal y.sup.2 n is subjected by a root extractor 3. The root extractor 3 comprises a second adding element 440 that receives the squared energy signal y.sup.2 n and outputs the desired energy signal yn at its output. To calculate the energy signal yn, adding element 440 adds the squared energy signal to two additional signals. These are firstly the energy signal yn-1fed back through a second state memory 450 from its own output and secondly a signal y.sup.2 n-1that is obtained by squaring and negating from the fed-back energy signal yn-1.
The conventional calculation of the energy signal yn shown here suffers from the following disadvantages:
By squaring the signal sn, its dynamic range is sharply increased. It is only possible to store the squared signal in a memory with a very large word width.
The square root routines used in conventional extraction of square roots converge slowly, often as a function of the magnitude of the amplitude of their input signal. They are therefore unsuitable for use in systems that require rapid convergence behavior of the detector, such as compander-expander systems for example.